1. Field of the Invention
The present invention relates to pre-insulated piping systems, and more specifically to a method for anticipating and selectively controlling the disbondment of the foam from the carrier pipe which may occur as these systems thermally expand in the presence of high temperature fluids being conveyed.
2. Description of the Prior Art
There are many instances in which insulated pipelines are needed. For example, distributed HVAC (heating, ventilation and air conditioning) applications utilize chilled water for cooling and steam for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam are distributed to other locations. For example, on a school campus, the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings.
A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulated pipelines is used to carry the steam from the boiler to the other locations and back to the boiler. The insulated pipelines are usually located underground.
Insulated pipe is conventional and commercially available. There are predominately two types of piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate “bonded” foam systems. Both of these systems use an inner carrier pipe to convey fluid. Although steel is commonly used for the inner pipe which carries the media to be piped, copper or aluminum or other metals as well as fiberglass, PVC, and similar materials may be utilized, as well. The present application is directed toward the “bonded” foam type system. These systems utilize a steel pipe to convey fluid. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyisocyanurate foam. Around the outside of the foam is a jacket of hard thermoplastic (such as high density polyethylene, HDPE). The foam has set up or cured within the outer jacket so as to bond to the jacket and to the inner pipe. The plastic jacket protects the foam from mechanical damage and also provides a water tight seal to prevent corrosion of the steel pipe. In the bonded type system, the foam and outer jacket do not move relative to the inner pipe. In the Class-A type system, on the other hand, the insulated inner pipe is designed to move independently of the associated outer jacket. In fact, there is an air gap between the inner pipe and outer carrier pipe in the class-A type system.
The most important engineering criteria for the traditional “bonded” foam type system is that it must be treated as a monolithic system. In other words, the foam is bonded to both the carrier pipe and the outer jacket. Therefore, the bonded system has traditionally been designed to move as a unit underground. Higher temperatures can act adversely upon the bonded foam system, however. The hot fluid in the steel carrier pipe causes the carrier pipe to thermally expand. At temperatures of 400° F. this expansion is on the order of 2.8 inches per 100 feet of pipe. This expansion is not a problem as long as the system remains bonded and the carrier pipe, foam and jacket move together as one unit underground. This movement is controlled by the expansion force of the steel carrier pipe, but it is the bond strength of the foam to the pipe and jacket that is important in keeping the system moving together. This monolithic movement of the system occurs along each incremental length of a particular run, and as long as total movement is not greater than 4 to 6 inches and the system remains bonded, no undue stress is subjected at any one point of the jacket. If the system however were to disbond, the surrounding soil would fix the jacket in place and the carrier pipe would still thermally expand thereby pushing thorough and destroying the jacket at the first change of direction.
Generally speaking, the proper choice of insulating materials can counteract many of the thermal expansion effects discussed above. It has been well established by industry case history that the polyurethane foam bond for systems running at 250° to 300° is strong enough to keep the entire system acting as a bonded system. However, for systems running above these temperatures a higher temperature rated foam, such as polyisocyanurate foam, is required. However, even in systems utilizing “high temperature” polyiscyanurate foam, the higher heat can in some circumstances, begin to fry the foam at the foam/carrier pipe interface, thereby bringing into question the strength of the foam bond to the steel carrier pipe.
Despite the advances which have been made in addressing the above problems, a need exists for improvements in pre-insulated piping systems which will either ensure that the insulating foam remains bonded to the carrier pipe, or which will ensure that the foam bond fails in a predictable manner and at preselected locations.
A need exists for an alternative fo the traditional “bonded” foam system which can be utilized at temperatures exceeding 250° F. to 350° F. without risk of having the carrier pipe rupture the outer jacket at changes of direction.
A need continues to exist for a pre-insulated piping system of the above type which effectively either prevents or accommodates foam disbondment, even at temperatures above 250 to 300° F.
A need also exists for a complimentary mechanical expansion component for such systems which compliments the bonding system and which is activated in the case of movement of the inner steel carrier pipe relative to the foam insulation to prevent the carrier pipe foam pushing through the outer jacket or causing other structural damage to the system.